So he has a crush on someone.
That doesn't mean he is poly. Or he IS poly, and wants to be with YOU. Just... has feelings of crushy he wants to share and express because YOU are his partner.
But just because he feels crushes doesn't mean he wants to act on it or do anything about it other than share his feelings with YOU, his partner.
Most of the times when I have crushes they go away in time because I don't do anything about other than tell my spouse. He laughs, teases me, and in time the crush fades because there's nothing real to sustain it.
What matters in YOUR agreement with your BF for your rship. If it is Closed, it is Closed. You do not have to Open it just because he experiences some crushes.